In single rotor helicopters utilizing fully articulated rotor blades there are three degrees of articulation. This permits the blades to change pitch, flap and hunt. Basically, pitch change is the only action controlled directly by the pilot, being controlled through cyclic and collective pitch sticks. Pitch change is accomplished by rotating the blade sleeve about a spindle and an associated thrust bearing assembly. A change in cyclic pitch causes the blades to flap which factor requires the use of flapping hinges. A flapping hinge in a tail rotor application normally is subjected to about .+-.4.degree. of blade flap and does not have the range of rotational movement such as that to which the pitch bearing is subjected.
The flapping hinge bearing, particularly as applied to a stiff in-plane tail rotor with no lag hinge, must provide a specific stiffness, especially in the edgewise plane. Further, it is important that blade edgewise and flatwise frequencies be separated. Coalescence of edgewise/flatwise frequencies must be avoided or extremely high stresses would result, leading to structural failure of the rotor assembly. Frequency tuning by spring rate adjustment of the flapping hinge bearing through the bearing structure of this invention can provide reduction of aerodynamic loading to lower blade stresses and minimize the up weight and flight weight. Also, the bearing geometry will afford the lowest possible blade weight and best frequency situation.
In certain helicopter articular rotor head models needle bearings typically have been used as the tail rotor flap bearing. However, due to current incorporation of composite blades with higher thrust requirements, bearing life has not been as long as desirable and more frequent replacement of the bearing has been required. Metal rolling element cylindrical or conical bearings require a lubrication system involving seals to contain the lubricant. Further, bearings of that type function best with large angular excursion, i.e., complete revolutions of the needle bearing. Conversely, they function poorly with small angular motion such as the flapping motion of an articulated rotor blade. The reason for this is the line contact and resultant high Hertz stress due to rollers merely rocking in place. This results in "fretting corrosion" which, as it progresses, causes the bearings to lose preload and eventually to wobble.
Replacement of metal rolling element bearings with radial elastomeric bearings has overcome the problems with metal bearings primarily because small amplitude flapping angles are ideal for the elastomeric type bearing. Replacement of the metal needle bearing with an elastomeric bearing results in dramatically extended bearing life, improves edgewise stiffness, requires no seals, and minimizes maintenance.
A frusto-conical bearing construction of concave conical form having bonded thin solid film lubricant coatings on multiple laminar surfaces with provision for axial adjustment of the inner race is described in Haines U.S. Pat. No. 3,984,152. An elastomeric bearing of convex conical form for helicopter rotors, having provision for exerting pressure on the assembly is described in Boppes et al U.S. Pat. No. 4,395,143.